Ultrasonic staking is used in the automotive industry (as well as in other industries) to attach plastic components together. There are usually many points to be staked in order to make a finished part, sometimes one hundred or more.
Typically, ultrasonic staking machine builders normally build a switching system with high voltage relays to use one ultrasonic supply and several converters. In a multiplexed fashion, one converter is switched in and an ultrasonic mechanism is turned on to weld the first point, then successively the next converter is switched in and the ultrasonic mechanism is again turned on to weld the second point. This is repeated multiple times as necessary to finish all points to be welded on the dashboard or other component.
This multiplexed arrangement is desired because of the number of points to be welded. There is a tradeoff between speed and the number of power supplies applied to the project. More supplies will cost more and reduce the time to complete the task; fewer supplies will reduce the cost but add time to the task. It is very desirable to use the least number of supplies that will meet the time requirements and minimize the costs for components.
Power supply cost is directly related to the power rating of the supply. Higher wattage means higher cost. It is also desirable to use the lowest rated power supply as possible.
The ultrasonic heads are driven to the parts by stepper motor actuators (although air cylinders are also typically used). The speed of this movement is also important to completing the task on time. The faster the movement, the quicker the task is complete but increased power will be required from the supply. The applied force is proportional to the speed and heat available (i.e., power rating of the supply).
The application designer must carefully select the lowest power supply wattage, the fastest motor speed and the best number of multiplex channels to optimize the application cost and timing. At the same time, overloading of the power supplies must be avoided.
The usual solution to avoiding overloading of the power supplies is to add additional supplies or to move the motors a little slower so the force builds at a lower rate. Both options are undesirable resulting in higher cost or a longer time to complete the process. It is also undesirable to require a 1000-watt supply, for example, when in a common example 99% of the weld occurs at less than 100 watts.
Attempts have been made to obviate the problem of power supply overloading by shutting off the power supply and/or varying the power/frequency output of the power supply when certain conditions (such as an overload or impending overload condition).
For example, U.S. Pat. Nos. 5,846,377 to Frantz et al. and 5,435,863 to Frantz disclose ultrasonic processing apparatus wherein the motional amplitude of the horn is varied in response to the amount of power provided to a workpiece, among other possible parameters. In one specific embodiment, the apparatus is used for a staking operation creating a stronger bond and a more pleasing appearance than previously attained because of decreased splatter of thermoplastic material.
U.S. Pat. No. 7,225,965 to Johansen discloses an ultrasonic welding system in which weld energy or ultrasonic voltage is used to control the on/off state of an ultrasonic generator and/or flag bad or suspect parts.
U.S. Pat. No. 5,637,947 to Kising et al. discloses an ultrasonic welding system with frequency control based at least in part upon a measured current, voltage and/or power.
U.S. Pat. No. 5,366,580 to Czach discloses a high frequency welding machine which is tuned to maintain a constant output power and die temperature in response to various sensed parameters, including plate current and plate current overload, among others.
U.S. Pat. No. 5,213,249 to Long et al. discloses an ultrasonic bonding system that monitors adhesion and/or dehesion between two substrates by sensing voltage and current supplied to an ultrasonic source, and varies or turns on/off the power to the source in response thereto.
U.S. Pat. No. 4,746,051 to Peter discloses an ultrasonic welding apparatus that controls termination of welding and/or that evaluates the quality of welded parts based upon time period and energy level measured from the point in time where welding power exceeds a power threshold.
U.S. Pat. No. 3,784,079 to Spanjer discloses an ultrasonic welding device that shuts off an ultrasonic power supply when a dip in voltage is detected.
Thus, the references described above employ voltage, current and/or power sensing in the context of an ultrasonic or similar device to turn power on/off, to vary frequency or amplitude and/or to evaluate the quality of a part. While these approaches may have some advantages over not providing any type of overload protection at all, they suffer from disadvantages of their own. Turning off the power supply completely would greatly add to the time necessary to complete the staking operation (particularly where overload conditions are encountered regularly), or may even make it impossible to complete the staking at all. On the other hand, varying the frequency or amplitude adds complexity to the ultrasonic mechanism and may shorten the life expectancy thereof.
Therefore, no ultrasonic staking system possessing satisfactory overload prevention is known in the prior art.